Sulphide Globules in a Porcellanite–Buchite Composite Xenolith from Stromboli Volcano (Aeolian Islands, Southern Italy): Products of Open-System Igneous Pyrometamorphism

Alberto Renzulli,Frank J Brink,Marco Taussi,Stefano Del Moro,Richard W Henley

doi:10.3390/min11060639

Abstract

Pyrometamorphism is the highest temperature end-member of the sanidinite facies (high-temperature, low-pressure contact metamorphism) and comprises both subsolidus and partial melting reactions which may locally lead to cryptocrystalline-glassy rocks (i.e., porcellanites and buchites). A wide range of pyrometamorphic ejecta, with different protoliths from Stromboli volcano, have been investigated over the last two decades. Among these, a heterogeneous (composite) glassy sample (B1) containing intimately mingled porcellanite and buchite lithotypes was selected to be studied through new FESEM–EDX and QEMSCAN™ mineral mapping investigations, coupled with the already available bulk rock composition data. This xenolith was chosen because of the unique and intriguing presence of abundant Cu–Fe sulphide globules within the buchite glass in contrast with the well-known general absence of sulphides in Stromboli basalts or their subvolcanic counterparts (dolerites) due to the oxygen fugacity of NNO + 0.5–NNO + 1 (or slightly lower) during magma crystallization. The investigated sample was ejected during the Stromboli paroxysm of 5 April 2003 when low porphyritic (LP) and high porphyritic (HP) basalts were erupted together. Both types of magmas are present as coatings of the porcellanite–buchite sample and were responsible for the last syn-eruptive xenoliths’s rim made of a thin crystalline-glassy selvage. The complex petrogenetic history of the B1 pyrometamorphic xenolith is tentatively explained in the framework of the shallow subvolcanic processes and vent system dynamics occurred shortly before (January–March 2003) the 5 April 2003 paroxysm. A multistep petrogenesis is proposed to account for the unique occurrence of sulphide globules in this composite pyrometamorphic xenolith. The initial stage is the pyrometamorphism of an already hydrothermally leached extrusive/subvolcanic vent system wall rock within the shallow volcano edifice. Successively, fragments of this wall rock were subject to further heating by continuous gas flux and interaction with Stromboli HP basalt at temperatures above 1000 °C to partially melt the xenolith. This is an open system process involving continuous exchange of volatile components between the gas flux and the evolving silicate melt. It is suggested that the reaction of plagioclase and ferromagnesian phenocrysts with SO2 and HCl from the volcanic gas during diffusion into the melt led to the formation of molecular CaCl in the melt, which then was released to the general gas flux. Sulphide formation is the consequence of the reaction of HCl dissolved into the melt from the gas phase, resulting in the release of H2 into the melt and lowering of fO2 driving reduction of the dissolved SO2.

Highlights

Pyrometamorphism is a type of thermal metamorphism involving very high temperatures—often to the point of melting in suitable lithologies at low pressures—that has been extensively reviewed by Grapes [1]
With respect to analysis of the context of the evolution of the composite xenolith, it is important to note that the basalt coating the xenolith is represented by the two juvenile magmas commonly erupted during paroxystic eruptions of Stromboli, namely low porphyritic (LP) and high porphyritic (HP) basalts (Figure 2)
We suggest that open-system igneous pyrometamorphism led to the formation of the immiscible sulphide globules

Summary

Introduction

Pyrometamorphism is a type of thermal metamorphism involving very high temperatures—often to the point of melting in suitable lithologies at low pressures—that has been extensively reviewed by Grapes [1]. Pyrometamorphic rocks are micro-cryptocrystalline to glassy, as the result of processes of recrystallization and melting/crystallization under high temperature/low pressure conditions. The high temperatures can be provided by magma (i.e., igneous pyrometamorphism), spontaneous combustion of coal, carbonaceous sediments, oil and gas (giving rise to the so called paralavas), or lightning that strikes at the surface. All of these conditions characterize the sanidinite facies of metamorphism (Figure 1). Applied to partially melted sandstones [11], buchite was later used to describe fused pelitic rocks [12]

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